When Awdry was writing, Diesel trains were replacing Steam trains, and a lot of train enthusiasts Were Very Upset. So on the one hand, I think you totally could read class into that (especially as, at least in the show, diesel trains and trucks are given a fairly distinctly low-class voice), but on the other hand, it was something that was going on that Awdry was concerned about.

There were a very large amount of trains pre-mass-marketing/toys, however more trains have been added AND existing trains get different (often wacky) paint jobs to push more toys. So there totally is "buy all our playsets and toys" going on now.